Electrical coil assemblies exist for numerous applications. In one application, that of voltage step-up or step-down, an induced voltage of one potential in a primary coil is stepped-up or stepped-down by a secondary coil as a function of the turns ratio. High voltage transformers of relatively small size exist both in communications equipment, such as televisions and the like, and in ignition systems for automobiles and the like. In each instance a relatively low potential applied to a primary coil is stepped-up to a relatively high voltage, i.e. thousands of volts, by a secondary winding.
In the design of such high-voltage coil assemblies as exist in the secondary of an automotive ignition transformer, it is necessary to design against electrical shorting or arcing in order to ensure the long life and integrity of the coil. Because each coil-turn in the secondary develops a potential thereacross and because such high voltage secondaries may include a very large number of turns, and thus a large total potential across the entire coil, care must be taken to minimize electrical shorting between coil-turns. Normally, the wire used in winding a coil will include a thin, insulative coating which may be rated for several hundred volts. In the winding of such a coil, it is necessary that a coil-turn of a relatively low potential not be in contact with a coil-turn of a substantially higher potential. To this end, the bobbin structure for the secondaries of high voltage transformers have been provided with multiple slots which effectively provide numerous small coils of limited axial extent. Examples of such coil assemblies are depicted in U.S. Pat. No. 4,274,136 to Onodera et al; U.S. Pat. No. 4,388,568 to Goseberg et al and PCT Application No. DE83-00184 of Worz having International Publication No. WO84/02224.
By employing the winding configuration of the aforementioned patents, and assuming for example that each coil-turn develops a potential thereacross of five volts and that ten such coil-turns exist in a particular layer in each slot on the bobbin, then each adjacent coil-turn in a particular layer in a slot will differ by only five volts from that of the preceding or following coil-turn and the coil-turns in the layer immediately above or below will typically differ in potential by only about 50 volts.
While the provision of slots in the bobbin does provide a plurality of coils of limited axial extent and thus limited difference in the potential between successive layers of coil-turns within a slot, there remains the possible problem that the coil wire which normally transitions or crosses-over from the uppermost layer of coil-turns in one slot to the lowermost layer of coil-turns in the next adjacent slot will be placed in undesirable proximity or contact with some of the radially uppermost or outermost turns in the second slot which have a greatly different electrical potential. Moreover, even in situations in which the difference in electrical potential is far less, the insulating coating on the coil wire which crosses over from one slot to the next may be subjected to considerable abrasion by the coil windings as they are subsequently formed. The aforementioned U.S. Pat. No. 4,274,136 discloses the use of notches or recesses in the flanges which constitute the walls to the successive slots. These recesses extend axially the full way through a respective flange and radially from the outer surface of the bobbin spindle to the radially outer end of the flange. While the provision of such recesses does enable the coil wire to transition from one slot to the next, it does not appear to provide particularly good separation or isolation of the transitioning coil wire from either the outermost coil-turns in the slot into which it is transitioning or the inward coil-turns in the slot from which it is originating. In this latter regard, when the coil-turns are completed in one slot and the transition is made from that slot to the bobbin spindle in the next adjacent slot via the notched recesses of U.S. Pat. No. 4,274,136, there exists the possibility that the winding tension on the wire will cause the transitioning wire to be pulled radially downward between the coil and the flange of the slot from which it is transitioning. In such instance, it will be appreciated that the aforementioned problem is again created, however, in this instance in a reversed manner.
Examples of coil assemblies which do provide grooves in the flanges separating adjacent coil segments include U.S. Pat. No. 3,661,342 to Sears and U.S. Pat. No. 2,355,477 to Stahl. In U.S. Pat. No. 3,661,342 there is disclosed a rectangular core having rectangular flanges and a corresponding cross-over groove between successive slots on the bobbin. However, the cross-over slot is formed by a complex contouring of the flange which defines an "enclosed" cross-over path, such that the coil winding is trapped axially, or longitudinally, of the bobbin as it transitions from one slot to the next.
The U.S. Pat. No. 2,355,477 discloses a multi-slot coil form of circular or polygonal configuration. The flanges between successive slots on the coil form are provided with grooves (or slots 15) in which the windings transitioning from one slot to the next may lie. Those grooves (slots 15) are directly open in an axial direction. However, the depth of those grooves is shallow relative to the greater axial width of the slots.
As used in the present application, the term "slot" denotes the recess extending radially inward of the bobbin's perimeter and into which multiple coil-turns are deposited, and the term "groove" denotes the generally linear recess formed in a respective flange and which provides a cross-over path for a winding between successive slots.
Recently, U.S. Pat. No. 4,638,282 by Ellison and assigned to United Technologies Automotive, Inc., the same inventor and assignee as in the present application, disclosed an improved wire cross-over arrangement for a coil assembly. In that patent a cross-over groove of relatively simple axially-open construction is provided in the wall of successive flanges to define a cross-over path for the wire transitioning from one slot to the next. More specifically, that patent discloses a specific geometry involving both the cross-over grooves and the slots for accommodating a "worst-case" winding condition. Specifically, the slots are relatively narrow and the axial depth of the cross-over grooves is sufficiently deep that even if the lead wire from a supply spool is in contact with the interior face of the opposite flange during the initial turn in a slot, the wire will remain substantially entirely in the cross-over groove. This feature is highly desirable for the purpose of minimizing or eliminating contact of the transitioning coil with subsequent coil turns in a slot which may be of relatively higher potential. For the generally circular bobbin geometry described in that patent, to attain the aforementioned characteristic while retaining the full structural integrity of the separating flanges it was generally necessary that the axial thickness of a flange be substantially as great as the width of a slot or conversely, that the width of a slot be relatively narrow. A shortcoming in such construction resides in the relatively large amount of material required for the flanges and the relatively small remaining space for the slots in which the coil turns are deposited. This may increase the overall volume and/or cost of the product.